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# grounding + 24vdc ? Answered

Can someone explain the following, I got 24 VDC power supply (or take 1.5V AA battery) which is connected to 110VAC outlet on the input, potential difference out of the power supply is 24VDC node (+) to node (-) and potential difference of either one to ground is 0V (outlet ground). Now I take wire and ground (+) of the 24 VDC to outlet ground. Questions:
1. Why i am still measuring 24 VDC across (+) and (-) nodes ? If ground (call it whatever common) is 0 v and ground is touching (+) which was prior to shorting it to ground raised in potential by 24 V in relation to (-) that means (-) is zero potential? Or is it + 24VDC and - 24VDC but than potential difference is 48V at the output of the power supply.
2. Does that mean that I raised the potential of the ground to 24V? Probably not
Thanks

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## 7 Replies

Downunder35m (author)2016-10-20

Before you kill yourself you should learn about electrical safety.
What comes out of a power supply is isolated from your mains power, so no chance to "ground" it.
Just the idea of trying this wrong as if it would have worked you would have caused a dead short.
I don't want to know why you tried this, do I?

onaumov (author)2016-10-20

We have ungrounded DC control circuit at work which utilizes ground detector lights which are two pilot lights wired in series across the + and - 24Vdc of power supply and the center point between the two lights is then grounded. My understanding whichever side gets grounded the light goes dim on that side which what we experiencing right now. In our case the dim light on +24 vdc out of power supply side which makes the ground to the - of the power supply light at potential of 24vdc by measuring with multimeter. That is why my question above. You apperently havent seen something like this ?

Downunder35m (author)2016-10-20

I have worked a lot with these, if you would have mentioned a lab setup it might have been clear.
But as stated by Max to make these grounded supplies work you need a power supply designed and connected for this purpose.
The usual setup has a floating ground that means it is totally isolated from mains ground.
If the power supply is equipped with a seperated ground, which is required for a lab setup then this ground is connected to the lab ground connection - but in all cases I encountered never to the mains ground directly, always seperated or with big filter blocks...

-max- (author)2016-10-20

This is simply because the output voltage of the 24V supply is a measure of the voltage between the positive and negative output of the supply. The entire output stage is electrically and galvanically isolated from the wall for safety reasons.

So ideally no current should ever flow between the output of the supply and any live connection to the grid, and because the points are essentially not related, your meter will never measure any real voltage.

steveastrouk (author)2016-10-20

0V is not "ground", that's why. You have a floating power supply, whose 0V line you can CONNECT to ground, and make its 0V, 0V with respect to ground.

If you had two 24V supplies like this, you could connect them in series, take the link between the two, and connect THAT to ground, and now you have a 0V line, a +24V line, and a -24V line.

onaumov (author)2016-10-20

i understand that. But my question still the same why is there potential difference between grounded +24 and - node out of the power supply 24vdc. That is only possible if positive side got to zero volts due to grounding and - side got to -24 but if -24 is there originally than potential difference before grounding would have been 48 vdc unless the - side out of power supply was 0 v originally and than when + 24 got grounded it turned -24.

steveastrouk (author)2016-10-20

Exactly. You have redefined your reference points !